The use of non-volatile memory systems such as flash memory storage systems is increasing due to the compact physical size of such memory systems and the ability for non-volatile memory to be repetitively reprogrammed. The compact physical size of flash memory storage systems facilitates the use of such storage systems in devices which are becoming increasingly prevalent. Devices which use flash memory storage systems include, but are not limited to, digital cameras, digital camcorders, digital music players, handheld personal computers, and global positioning devices. The ability to repetitively reprogram non-volatile memory included in flash memory storage systems enables flash memory storage systems to be used and reused.
In general, flash memory storage systems may include flash memory cards and flash memory chip sets. Flash memory chip sets generally include flash memory components and a controller. Typically, a flash memory chip set may be arranged to be assembled into an embedded system. The manufacturers of such assemblies or host systems typically acquire flash memory in component-form, as well as other components, and then assemble the flash memory and the other components into a host system.
Although non-volatile memory or, more specifically, flash memory storage blocks within flash memory systems may be repetitively programmed and erased, each block or physical location may only be erased a certain number of times before the block wears out, i.e., before memory capacity begins to become smaller. That is, each block has a program and erase cycle limit. In some memories, a block may be erased up to approximately ten thousand times before the block is considered to be unusable. In other memories, a block may be erased up to approximately one hundred thousand times or even up to a million times before the block is considered to be worn out. When a block is worn out, thereby causing a loss of use or a significant degradation of performance to a portion of the overall storage volume of the flash memory system, a user of the flash memory system may be adversely affected, as for the example through the loss of programmed data or the inability to program data.
The wear on blocks, or physical locations, within a flash memory system varies depending upon how much each of the blocks is programmed. If a block or, more generally, a storage element, is programmed once, then effectively never reprogrammed, the number of program and erase cycles and, hence, wear associated with that block will generally be relatively low. However, if a block is repetitively written to and erased, e.g., cycled, the wear associated with that block will generally be relatively high. As logical block addresses (LBAs) are used by hosts, e.g., systems which access or use a flash memory system, to access data programmed in a flash memory system, if a host repeatedly uses the same LBAs to write and overwrite data, the same physical locations or blocks within the flash memory system are repeatedly written to and erased, as will be appreciated by those of skill in the art.
When some blocks are effectively worn out while other blocks are relatively unworn, the existence of the worn out blocks generally compromises the overall performance of the flash memory system. In addition to degradation of performance associated with worn out blocks themselves, the overall performance of the flash memory system may be compromised when an insufficient number of blocks which are not worn out are available to program desired data. Often, a flash memory system may be deemed unusable when a critical number worn out blocks are present in the flash memory system, even when many other cells in the flash memory system are relatively unworn. When a flash memory system which includes a substantial number of relatively unworn blocks is considered to be unusable, many resources associated with the flash memory system are effectively wasted.
In order to increase the likelihood that blocks within a flash memory system are worn fairly evenly, wear leveling operations are often performed. Wear leveling operations, as will be understood by those skilled in the art, are generally arranged to allow the physical locations or blocks which are associated with particular LBAs to be changed such that the same LBAs are not always associated with the same physical locations or blocks. By changing the block associations of LBAs, it is less likely that a particular block may wear out well before other blocks wear out.
Recently, non-volatile memories, having data transmitted in a group of blocks at once, are rapidly developing due to the fact that such non-volatile memory has an access speed faster than a traditional non-volatile memory which transmits data one block at a time. In other words, data in such non-volatile memory is accessed in the unit of memory units rather than blocks, and thus the access speed is increased. Hence, data is also erased in the unit of memory units. However, unused blocks of a memory unit may be erased concurrently and uneven use of the blocks of the memory unit may occur. Therefore, a subsystem wear leveling method for a non-volatile memory and a controller using the same are desperately desired so as to prolong a lifespan of the non-volatile memory.